The rotor system of a helicopter mounts and supports the helicopter blades to the engine output shaft and includes, among other things, a hub, which is mounted on the output shaft and a set of spindles or yokes, which attach the blades to the hub. The rotor system must withstand the tremendous centrifugal force the blades apply during rotation while permitting their flapping, pitch and lead/lag motions. The many different systems utilized for this task are variations on two basic designs, referred to as articulated and bearingless, or hingeless. The articulated system utilizes a rigid spindle equipped with hinges and bearings to facilitate the aforementioned blade motions. The bearingless system comprises special composite material spindles (referred to as flexbeams) that are flexible enough to twist and bend to allow blade movement without hinges, bearings and additional mechanics. The present invention is directed towards bearingless rotor flexbeams.
A certain minimum cross-section is required for a flexbeam to support centrifugal blade loads and static blade droop loads, while the aforementioned blade movements require the flexbeam to also have considerable torsional flexibility. The flexbeam cannot be too soft in chordwise and flapwise flexibility, though, because significant flapwise blade deformation, or buckling, will occur under normal operating conditions. Tradeoffs, therefore, are to be made between centrifugal loading strength, fatigue strength, torsional flexibility, chordwise and flapwise flexibility. Commonly owned U.S. Pat. No. 4,746,272 (Noehren et al.) discloses a flexbeam system which is indicative of recent designs addressing these tradeoffs. In Noehren et al., four helicopter blades are attached to two unitary composite flexbeams each holding diametrically opposing blades. The cross-section of each flexbeam is tailored along the length to separate the maximum strains into a flap flexure inboard section, a lag-torsion outboard section, and blade and hub attachment sections. The flap flexure section has a rectangular cross-section which tapers in thickness and is designed to accommodate blade flapping motion while the lag-torsion section has several different multiple-lobed H-beam cross-sections designed for lead-lag frequency placement, minimization of the torsion moment caused by blade twist, and minimization of chordwise flexibility.
Although prior flexbeam designs such as disclosed in Noehren et al have improved the usefulness of bearingless rotors, two specific drawbacks impose a significant deterrent to their utilization. One is that complex flexbeam designs such as multiple-lobed H-beams are susceptible to high stress concentrations within the structure, thereby causing a relatively short life span and a reliability problem. The other drawback being that prior flexbeams are difficult and costly to manufacture.
In addition, expanded mission profiles have fostered a demand on helicopters for an expanded flight envelope and increased payload. Reduction of rotor size and weight with increased strength and flexibility certainly help promote this cause. Of course, simplicity is also highly desirable. A new bearingless rotor design which eliminates the previously mentioned drawbacks and satisfies the new requirements is necessitated.